Volume limiter device



Nov. 27, 1956 A. E. WOODRUFF 2,772,390

VOLUME LIMITER DEVICE 2 Sheets-Sheet 1 Filed March 27, 1953 lrvllll IIIIIII I q Q -INVENTOR. ALBERT E. WOODRUFF BY WW ATTY.

Nov. 27, 1956 A. E. WOODRUFF 2,772,390

VOLUME LIMITER DEVICE Filed March 27, 1953 2 Sheets-Shae: 2

INVENTOR. ALBERT E. WOODRUFF ATTY.

United States VOLUME LllVIITER DEVICE Application March 27, 1953, Serial No. 345,067

6 Claims. (11. 323-66) This invention relates in general to signal transmission systems and more particularly to voice current volume limiter circuits for use in such systems in which voice currents from transmission lines are used as the modulating signals for radio transmitters.

In modulating a radio transmitter with speech currents from telephone line it is frequently found that there is a variation in the voice current energy levels of from 40 to 50 decibels due to atmospheric noise and other causes. If an amplifier is used to raise the voice current energy levels, the higher levels tend to produce an intolerable overloading of the transmitter. On the other hand, without such amplification the voice currents of the lower energy levels will be unheard above the noise of the circuit. Existing amplifiers designed to correct this situation do so by limiting the volume of the voice currents during periods of peak load. Many of these amplifiers, however, suifer from the common defect that there is maximum amplification when there is no speech energy being applied to the line. During these intervals of silence the amplifier adjusts itself to maximum sensitivity with the result that circuit noise is amplified to a disturbing level. Too, the first speech sounds after such intervals of silence are applied to the amplifier while it is in its sensitive condition with a distracting effect upon the listener and a general reduction in the intelligibility of speech.

The principal object, therefore, of this invention is to provide a volume limiter circuit having comparatively little sensitivity to voice currents of low energy levels and comparatively greater sensitivity to voice currents of high energy levels and which will return to a condition of average sensitivity when no voice current is introduced into it.

It is another object of thi invention to provide a volume limiter circuit capable of a rapid shift from one degree of sensitivity to another.

A further object of this invention is to provide a volume limiter circuit utilizing a cathode-ray beam to select the value of the volume limiting voltage.

Still another object of this invention is to provide a volume limiter circuit in which cathode produced electrons of a cathode-ray'tube are used directly in the circuit to produce the volume limiting voltage.

Another object of this invention is to provide a volume limiter circuit utilizing a plurality of resistors of consecutive values in connection with an associated plurality of target plates of a cathode-ray tube to producethe limiting voltages.

' An additional object of this invention is to provide a voltage selecting circuit including a multi-plate cathoderay tube for selecting the output voltage desired.

Other objectsand features of the invention will become apparent from the following specification and appended claims taken with reference to the accompanying drawings in which:

Fig. 1 shows a schematic circuit diagram of one embodiment of the invention wherein electrons of a cathodelimiting voltages;

' atent Fig. 2 shows a schematic circuit diagram of another embodiment of the invention wherein secondary emission electrons of the cathode-ray tube are used in a separate circuit to produce the volume limiting voltages; and

Fig. 3 shows a schematic circuit diagram of still another embodiment of the invention wherein the voice current input is impressed directly across the volume limiter voltage load resistor.

In the drawings corresponding parts of identical elements and identical elements performing the same function in each embodiment have been designated by the same numerals.

In a volume limiter circuit constructed in accordance with the invention, a portion of the input voice current energy is rectified after proper amplification and the direct current voltage thus obtained is impressed across a single pair of deflection plates of a cathode-ray tube. The cathode-ray tube used in the invention is so constructed that the cathode produced electron beam may be directed at any one of a plurality of insulated target plates placed in the sweep path of the beam, the particular target plate selected depending upon the magnitude of the beam deflecting potential. The cathode produced electron beam may be directly used in the volume limiter circuit as in the embodiment of Fig. 1, in which case a plate-tocathode return path is provided for each plate. Each target plate has included in its circuit a resistance element across which a voltage to be used'for limiting the input signals is developed. Since each such plate resistor will be assigned a difierent value, the limiting voltage will. vary in value depending upon the target plate selected by the electron beam as controlled by the deflection potential. Thus, the resistance values are so selected that when an input signal of a particular magnitude is rectified and, by means of the deflection plates, causes a deflection in the electron beam, the limiting voltage desired will be determined by the resistor of the target plate struck by the beam.

A variable impedance element, preferably in the form of a bridge, is placed across the transmission line to limit the energy level of the voice current signals. The bridge is composed of elements of non-linear resistance characteristics as, for example, germanium crystal elements. Two vertices of the bridge are connected across the line while the other two vertices are connected to the control circuit. The limiting voltage developed across the target plate resistors provides a direct current polarizing potential for the bridge by which the current flow through the bridge is controlled. When no polarizing potential is present the resistance of the bridge is high and the loss on the line is negligible, that is, the input signal passes along the line substantially unchanged. As the polarizing potential is increased as controlled by the increased line voltage, the resistance of the bridge is decreased and the attenuation of the input signal across the bridge is increased.

Average sensitivity of the circuit during intervals of no input signals can be achieved as shown in Fig. 1,.

by omitting the resistor of the target plate struck by the electron beam during this interval and connecting this plate to the plate having a resistor designed for average sensitivity.

Referring now more particularly to the embodiment of the invention as represented by Fig. 1, a variable impedance bridge 16 consisting of non-linear resistance elements 11 is connected to incoming conductors land 5 i by means of transformer 8 and to outgoing conductors by means of transformer 9. Two vertices of the bridge are connected across the line and the other two vertices are connected across the volume limiter load resistor 15.

A portion of the input signal voltage is applied to amplifier 13, and after amplification to a convenient value, is rectified by rectifier 14 and impressed across load resistor 16. The filtering effect of capacitor 17 shunted across load resistor 16v acts to level the pulsating direct currentoutput of rectifier 14. This voltage across resistor 16, varying in magnitude with, and proportional to, the changing power level of the input voice signals constitutes the control voltage for cathode-ray tube 12.

A single pair of deflection plates 24 of cathode-ray tube 12 are connected in parallel with resistor so that the potentials on plates 24 vary as the voltage across resistor 16. Cathode-ray tube 12 is provided with a conventional electron gun 40 the elements of which receive their operating potentials from taps 27 of voltage divider 22 placed across a suitable voltage source 43. A suitable source of filament heating current may be connected to terminals 19 and 20.

Cathode-ray tube 12 is provided with a plurality of insulated target plates 28, 29, of which only the first and last shown are numbered. The plates, preferably not fewer than thirteen, are so positioned as to lie in the sweep plane of the electron beam. The electron beam will therefore strike one of the plates 28 to 29, the plate struck being determined by the magnitude of the control voltage acting on plates 24 which in turn is determined by the input voltage to the amplifier. A biasing voltage 23 is provided to maintain the electron beam on first plate 28 when the control voltage is of a zero value. Thus when the opposing control voltage increases from a zero to a maximum value, the electron beam moves across the series of target plates from the first plate 28 to the last plate 29. Voltage source 26 provides a positive potential for each of the plates 28 to 29. Each plate, with the exception of first plate 28 in this embodiment, is provided with a load resistor 37 to 38 in its circuit, a further common voltage dividing load resistor 15 completing the plate-to-cathode circuit which may be traced as follows: the plate struck by the electron beam, its resistor 37 to 38, voltage source 26, resistor 15, voltage source 41, terminal 27, cathode and electron beam. The values of resistances 37 to 38 are so determined that the potential across load resistor 15 will vary in any pro-determined manner as the electron beam sweeps across the series of insulated plates. Plates 28 to 29 are placed in an overlapping manner to prevent the opening of the circuit in the event that the electron beam should be momentarily directed between two plates. An extension 34 to the last plate 29 is provided to permit the control voltage necessary to deflect the electron beam to the last plate 29 to be exceeded without opening the circuit.

An important feature of this invention in which the limiting effect for a zero input signal will be that of the limiting effect for a signal of average power is ac.- complished in the embodiment of Fig. 1 by omitting the resistor from the first plate 28 and connecting this plate to the plate having a resistor whose value has been determined as effecting an average limiting action. It is to be understood that the same result may be achieved by inserting in the circuit of the first plate 28 in this embodiment a resistor of the value equal to that of the resistor accomplishing an average limiting action.

The limiting voltage to be applied to two vertices of variable impedance bridge as a polarizing potential is developed across load resistor 15. The values of the resistances 37 to 38 are chosen such that the voltage drop across resistor will remain essentially constant within the limits of the number of steps provided by the target plates as the control voltage varies between values of the order of one to 100 volts D. C. The filter effect of capacitor 18 serves to smooth out the direct current voltage. supplied to the bridge 10.

If a signal of an exceptionally high energy level is introduced into the present invention the voltage drop across resistor 16 will be relatively large and will be in opposition to the deflection plate biasing voltage 23. As the potential difference between plates 24 reverses in polarity clue to the interaction of voltage 23 and the increasing voltage across resistor 16, the electron beam will be caused to move across the series of target plates. Assuming a maximum input energy peak, the beam will rest on plate 29 or its extension 34. The value of resist-. ance 38 is chosen such that the voltage across voltage dividing load resistor 15 will now be at a maximum thus increasing the polarizing potential on bridge 10 thereby decreasing its resistance with a resulting increase of attenuation on the line.

Fig. 2 of the drawings shows an embodiment substantially similar in structure and operation to that of Fig. 1 with the exception that use is made of the secondary electron emission of the target plates. In this embodiment a greater efficiency is achieved through the greater current caused to flow through voltage dividing load resistor 15. This greater current flow is the result of the favorable ratio of the number of secondary electrons dislodged from the target plate by the direct electron beam to the number of electrons absorbed by the plate. Cathode-ray tube 12 in this case is identical to the tube of Fig. 1 except that a grid 32 is provided having an extension 33. An appropriate voltage source 26 supplies a positive potential to grid 32 which then attracts any secondary electrons dislodged from a negatively charged target plate by the impinging electron beam. As in the embodiment of Fig. 1, the limiting voltage is developed across load resistor 15 and applied, as previously de-- scribed for Fig. 1, to a variable impedance bridge 10.

The external circuit of the cathode-ray tube may be traced as follows: any one of plates 28 to 29, grid 32, voltage source 26, and corresponding plate resistor-'37 to 38. It should be noted that in this embodiment average sensitivity in the event of no input signal is achieved plates 28 to 29, and plate resistors 37 to 38, identical tothat of the embodiment of Fig. 2. An external plate circuit is also provided with the limiting voltage developed across voltage divider load resistor 15. In this embodiment, however, attenuation of signals of high energy level is not accomplished by means of a variable impedance bridge as in the embodiments of Figs. 1 and 2. A secondary winding of transformer 27 is inserted inthe external circuit in series with voltage source 26 and load resistor 15. The primary winding of transformer 27 is connected to input terminals 4 and 5 in such a manner that the incoming voice frequency voltage is superimposed upon the potential ofbattery 26. A portion of the line voltageintroduced to transformer 27' will appear across load resistor 15 and the values ofplate resistors 37 to 38 can be so determined that this A. C. voltage will be of a con-.

stant value as controlled by the action of the limitingcircuit. The action of the electron beam in producing a limiting voltage is identical to that described for the embodiment of Fig. 2. The output signal will appear across output terminals 6 and 7.

Although particular embodiments of the cuit and arrangement and location of the parts may be made without departing from the spirit and scope of the invention as defined in the appended claims.

' What is claimed is:

1. A voltage regulating device comprising: a circuit in cluding a load resistor, a source of: current forcausing avoltage drop across said load resistor, means. for varying? invention have 7 been described and illustrated, modifications in the cir-' said voltage drop comprising a plurality of resistance elements having difierent resistance values, one terminus of each of said resistance elements connected to said load resistor, and means for selectively including any one of said plurality of resistance elements with said load resistor in said circuit, said last-mentioned means comprising a space discharge device having means for producing a stream of electrons and having a plurality of single insulated target plates in the sweep path of said electron stream, each of said plates connected to the other terminus of each of said resistance elements, respectively, said stream of electrons completing said circuit through any one of said resistance elements, and deflecting means for deflecting said electron stream from one of said target plates to another to thereby regulate said voltage drop across said load resistor in accordance only with said individual resistance values.

2. A voltage regulating device comprising a cathoderay tube having means for producing an electron beam and having a plurality of insulated target plates in the sweep path of said beam, each of said plates having connected thereto an individual resistance element, said resistance elements having individually predetermined values, an operating circuit including a load resistor, each one of said target plates connected through its connected resistance element to one end of said load resistor, means for deflecting said electron beam from one of said target plates to another, contact of said electron beam with any one of said target plates causing said one target plate to be connected through said beam to the other end of said load resistor, thereby completing said circuit through the resistance element connected to said one target plate causing a current in said circuit whereby a potential is developed across said load resistor, the magnitude of said potential being dependent only upon said predetermined value of said connected resistance element, deflection of said electron beam from one of said target plates to another operative to vary said potential developed across said load resistor by said current in said operating circuit according to the respective values of said connected resistance elements.

3. A voltage regulating device according to claim 2, said means for deflecting said electron beam including a signal input circuit, said deflecting means operative responsive to voltage changes in said input circuit to deflect said electron beam from one of said plurality of target plates to another in accordance with the signal level on said signal input circuit whereby said potential developed across said load resistor by said current in said operating circuit including said load resistor is varied according to the respective values of said connected resistance elements.

4. A voltage regulating device according to claim 3 in which said predetermined resistance values of said individual resistance elements are selected such that signals of high energy levels applied to said signal input circuit will eflect a progressive limiting of said voltage developed across said load resistor as said electron beam is caused to move across said plurality of target plates.

5. A voltage regulating device according to claim 4 in which the value of the resistance element connected to the target plate in the path of said electron beam when said beam is not being affected by said input circuit is selected such that said potential across said load resistor will be of an average value with respect to the values of said potential when said beam is directed at said other target plates.

6. A voltage regulating device comprising a cathoderay tube having means for producing a stream of electrons, a target comprising a plurality of insulated plates in the sweep path of said stream, each of said plates being capable of secondary emission when struck by said electron stream, each of said plates having connected thereto an individual resistance element, each of said resistance elements having a separately determined resistance value, and a grid element associated with said target plates and extending immediately opposite said plates; a circuit including a load resistor and a source of potential, said grid connected to one end of said load resistor, each of said plates connected through its connected resistance element to the other end of said load resistor, said source of potential providing a positive potential for said grid and a negative potential for said plates, said secondary emission causing a current to flow in said circuit from said grid to the one of said plates struck by said electron stream whereby a potential is developed across said load resistor, and means operative responsive to signals of varying energy levels for deflecting said electron stream in accordance with said varying energy levels from one of said plurality of plates to another whereby said potential developed across said load resistor is varied in accordance with said separately determined resistance values.

References Cited in the file of this patent UNITED STATES PATENTS 2,003,428 Cowan June 4, 1935 2,208,923 Curtis July 23, 1940 2,588,292 Rittner et al. Mar. 4, 1952 2,589,704 Kirkpatrick et al. Mar. 18, 1952 FOREIGN PATENTS 450,040 Germany Sept. 27, 1927 

